The latest Queens Park Rangers transfer news, views and rumours.
The 19-year-old Canadian made his senior debut at Barnsley on the final day of last season and will now play with Russel Slade's side until October.
"I'm very excited to come to the club. Orient did very well last season and caught my eye and I just want to help here in anyway I can," Petrasso told Leyton Orient's official website.
"I'm a very quick winger so hopefully can bring some pace to the team. I've had two loans in League 1 before which were great for me. This season I want to keep getting games under my belt and keep improving.
"I'm very excited for Saturday and hope that I can make my debut."
The Brazilian previously worked under Rs boss Harry Redknapp during his time in north London.
And the Argentine says it was not an easy decision to sell the Spurs midfielder but admits Tottenham simply had to reduce the size of their squad.
"You need to take a decision," Pochettino told reporters.
"It is true that we have a bigger squad and needed to reduce our squad – in these moments you need to make a decision but this is football."